1. Field of the Invention
The present invention relates to a method and apparatus for controlling the transmission power of a transmitter at an optimum level. More particularly, the present invention is concerned with a method and apparatus for controlling the transmission power of a transmitter of an earth station communicating through a communication satellite with another earth station in a satellite communication system so that the input power of a transponder mounted on the communication satellite attains an optimum level in spite of a variation in the attenuation ratio caused by rainfall, etc.
2. Description of the Related Art
In a satellite communication system, a radio wave transmitted from an earth station through an up link is relayed in a transponder mounted on a communication satellite, and is received through a down link in another earth station. Input power of the transponder is limited because power usable within the satellite is limited. In addition, if the input power level is too small, the power level in the down link becomes small and the received power level in the latter earth station attains a level below the system margin. Therefore, the input power of the transponder must be within an optimum range to maintain stable satellite communication.
An attenuation ratio in the up link and the down link is not constant but varies because of rainfall etc. The variation in the attenuation ratio is small in a lower frequency band such as C band (up link: 6 GHz; down link: 4 GHz) and is large in a higher frequency band such as Ku band (up link: 14 GHz; down link: 12 GHz) and Ka band (up link: 30 GHz; down link: 20 GHz. Therefore, if the Ku or Ka band is utilized, compensation for the rainfall attenuation in the up link, i.e. up link compensation is necessary for maintaining the input power of the transponder within the optimum range.
The up link compensation that has already been proposed is attained by increasing the transmission power of the former earth station in accordance with an evaluated increase of the attenuation factor, i.e., an evaluated rainfall attenuation factor in the up link.
The evaluation of the up link rainfall attenuation factor is performed by measuring received levels of a signal returned from the satellite and received levels of a beacon signal in rainy weather conditions and in clear weather conditions. The beacon signal is usually transmitted from the satellite at a frequency at the edge of a frequency band of the down link or in a different polarized wave from that of the main signals.
The rainfall attenuation factor in the up link and the down link in the returned signal are represented as L.sub.ur and L.sub.dr [dB], respectively, and the rainfall attenuation factor in the down link in the beacon signal is represented as L.sub.db. A summation of the rainfall attenuation factors L.sub.ur +L.sub.dr is determined by measuring levels of received return signals in clear weather conditions and in rainy weather conditions and by calculating the difference between the two levels. The rainfall attenuation factor L.sub.db is determined by measuring levels of received beacon signals in clear weather conditions and in rainy weather conditions and by calculating the difference between the two levels. Assuming L.sub.dr .apprxeq.L.sub.db, the up link rainfall attenuation factor L.sub.ur is calculated from the following formula. EQU L.sub.ur =(L.sub.ur +L.sub.dr)-L.sub.dr .apprxeq.(L.sub.ur +L.sub.dr)-L.sub.db
The transmission power of the earth station is controlled so as to compensate the up link attenuation factor L.sub.ur, to thereby maintain the input power of the transponder at an optimum level. In the case that the satellite does not transmit the beacon signal, a telemetry signal for watching the satellite can be used for estimating the up link attenuation factor L.sub.ur.
One of the most important problem in transmission power control of the earth station is control accuracy. The aforementioned conventional methods do not provide sufficient control accuracy because of the following factors:
i) a variation in transmission power of a beacon transmitter mounted on the satellite, PA1 ii) a variation in a level detector for the beacon signal, PA1 iii) a variation in a level detector for the return signal, PA1 iv) a gain difference between a beacon receiver and a receiver for receiving the return signal, PA1 v) a gain variation in the transponder, and PA1 vi) small signal suppressing effect in the transponder. PA1 generating a first signal and a second signal having a level different than the level of the first signal; PA1 transmitting the first and the second signals from the transmitter to the satellite; PA1 receiving return signals of the first and the second signals from the satellite; PA1 measuring a phase difference between the return signals of the first and the second signals; and PA1 controlling the transmission power so that the phase difference becomes equal to a predetermined value. PA1 means for generating a first signal and a second signal having a level different than the level of the first signal; PA1 means for transmitting the first and the second signals to the satellite; PA1 means for receiving return signals of the first and the second signals from the satellite; PA1 means for measuring a phase difference between the return signals of the first and the second signals; and PA1 means for controlling the transmission power so that the phase difference becomes equal to a predetermined value.,
A summation of the above deterioration factors amounts to, for example, 4 dB. Since the degree of deterioration in the control accuracy is comparable to the extent of the transmission power control, satellite communication is often more stable unless the transmission power control is carried out.
In addition, if the beacon signal is carried on a different polarized wave from that of the main signal, exclusive receiver equipment including a polarized wave branching circuit, a low noise amplifier, and a frequency converter is required, and therefore the equipment is large in construction compared to that for the main signal.
In order to improve said shortcomings, the Applicant has proposed a transmission power control method utilizing non-linearity in input output characteristics of the transponder, which is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 3-139027. In this method, the set point of the input power of the transponder is set to a value near the saturation level. Transmission signals having two different levels are transmitted to the satellite and transmission power is controlled so that a level difference in received return signals becomes equal to a predetermined value. When the input power of the transponder decreases because of rainfall attenuation; the level difference in the received return signals becomes large because the input power of the transponder falls below the saturation level. Thus, stabilization of the input power of the transponder is attained by controlling the transmission power so that the level difference in the return signals becomes equal to a value measured in clear weather conditions.
However, in the aforementioned method, since the set point of the input power of the transponder is set to a value near the saturation level, a problem exists in that large intermodulation distortion occurs. Furthermore, since a momentary power larger than a steady-state power is transmitted, it is difficult to realize said method because of legal restrictions.